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High temperature membrane bioreactor treating kraft evaporator condensate under steady and transient conditions Jen, Ruey-chiu

Abstract

Research results from Bérubé (2000) have shown that the treatment of kraft evaporator condensate for reuse using a high temperature membrane bioreactor (MBR) is not only technically feasible, but can also be economically attractive. However, dynamics in daily operating conditions at kraft mills would result in non-steady state loadings to such a treatment system. Hence, the influence of transient operating conditions on an MBR system requires investigation before the system could be considered for full-scale plants. Two bench-scale, high temperature MBRs (called RI for Reactor 1 and R2 for Reactor 2) were operated under conditions proposed by Bérubé (2000) to examine system performance under steady state operation. The operating parameters selected were as follows - 38-day sludge retention time (SRT), 9-hr hydraulic retention time (HRT), and evaporator condensate that contained 1,200 mg methanol /L. During the steady state experiment, the MBRs exhibited stable removal of the main contaminants. Removal efficiencies of 95 % for methanol and 64 % for organic components expressed as total organic carbon (TOC) were observed. Observed growth yields as low as 0.037 for RI and 0.025 for R2 were found. Effects of methanol shock loadings, black liquor spills, and pulp mill shutdown on a high temperature MBR treating condensate were the focus of the present research project. The reactors were subjected to four shock loadings to investigate long-term effects, and one shock loading to identify short-term effects. Results showed that the high temperature MBRs were reasonably stable and able to achieve the same removal efficiency when the load was increased by 1.5 and 2 times instantaneously. Overload of methanol was observed during the methanol shock loading test with 2.5 times the regular methanol concentration. However, the system recovered 4 hours after the short-term shock loadings, and two days after the long-term shock loadings. The MBRs started to shown inhibitory effects after the long-term black liquor carryover test with 8 mL black liquor per litre condensate. During the black liquor carryover test with 16 mL black liquor per litre condensate, methanol removal efficiency was greatly decreased and this negatively influenced TOC and chemical oxygen demand (COD) removal efficiencies. However, the system recovered 4 hours after the short-term shock loadings, and two days after the long-term shock loadings. During the tests, the colour of the MBR permeates remained relatively constant while the dissolved solids concentrations of the permeates increased slightly. Methanol, TOC, and COD utilization coefficients decreased during the shutdown period. However, the MBR system recovered along with resumption of loading fairly well. The MBR was capable of handling the 10-day shutdown period and recovered in 4 days to full capacity. No deleterious effects from 10-day shutdown were observed.

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